1. Field of the Invention
This invention relates generally to solid-state light-emitting devices, and more particularly to a semiconductor laser that is electronically tunable in the oscillation wavelength thereof, and a process of forming the same.
2. Description of the Related Art
As the complexity of communication processing architecture has increased, it has become more important to develop coherent optical communication systems of larger capacity and of longer effective communication distance. In particular, a coherent optical communication system employing a frequency multiplexed modulation scheme is very promising at present due to its significant increase in the amount of transmittable information.
In the frequency-multiplexed optical communication system, the receiver side (e.g., a subscriber terminal) is provided with a semiconductor laser as a light emitting/receiving device for selecting a desired communication channel. This laser is a light-emitting device which can change its own oscillation wavelength. In selecting the desired communication channel on the receiver side, the laser is electrically driven to vary the lasing wavelength to tune the oscillation frequency to that of the desired channel. Obviously, widening the wavelength tuning range may cause the tunable frequency band to be expanded desirably. However, conventional wavelength-tunable semiconductor lasers cannot fulfill the following two conflicting requirements: (1) widening the wavelength tuning range, and (2) narrowing the spectral linewidth of oscillation at the varied wavelength.
Conventionally, a multi-electrode distributed Bragg reflector (DBR) semiconductor laser is known which can vary the Bragg wavelength continuously, while the continuous variation amount is relatively large. The resultant linewidth however expands undesirably to range from 10 to 20 MHz in tuning the wavelength, making it impossible to attain the desired narrow linewidth for optical communication systems. The expansion of linewidth originates from the occurrence of carrier noise in the DBR region.
A multi-electrode distributed feedback (DFB) semiconductor laser is also known which has a good oscillating linewidth, and is thus a very promising light-emitting device for use in coherent optical transmission systems. Unfortunately, however, the laser suffers from a serious problem of an inherently narrow wavelength tuning range, as will be described below.
The laser has been described in M.C. Wu et al., "CLEO '90," at p. 667, wherein it has an active region with a uniform quantum well structure. Two electrodes are provided to inject current carriers into the active region. With this arrangement, the gain versus carrier density characteristic is non-linear. A required gain for oscillation may be obtained by selecting suitable regions of different differential gains and by optimizing the amount of carrier injection from each electrode. In other words, as the total carrier density is variable, it is possible to cause the refractive index and frequency (wavelength), which vary linearly to the carrier density, to be variable. In this case, the selectable differential-gain region is limited only to the region included within a specific range wherein an operating point is available on a single gain versus carrier density characteristic curve. This results in that the wavelength tuning range remains as little as 6 nanometers (nm) or less.